1. Field of the Invention
The invention relates to a nonvolatile memory devices and a driving method thereof.
2. Related Art
In recent years, microfabrication of nonvolatile memory devices such as NAND flash memories has increasingly proceeded. If a distance between adjacent memory cells is narrower to follow microfabrication of a device, proximity effect between the memory cells becomes conspicuous. The proximity effect is the phenomenon that a memory cell is interfered with an adjacent memory cell by capacitive coupling or the like between the memory cells. For example, charges accumulated in the adjacent memory cell influences data stored in the memory cell. If the distance between the memory cells adjacent to each other is narrower, the influence of the charges accumulated in the adjacent memory cell on the cell is greater.
To correctly read data stored in a memory cell, it is necessary that a threshold voltage of the memory cell falls within a predetermined range. Normally, right after data is written to the memory cell, the threshold voltage of the memory cell falls within the predetermined range. However, if data is written to a memory cell adjacent to the memory cell, the threshold voltage of the memory cell fluctuates by the proximity effect. This causes erroneous reading of data.
Meanwhile, if a distribution width of threshold voltages of memory cells during a data write operation is set small as measures against the proximity effect, a step width of a write voltage is required to be reduced accordingly. If the step width of the write voltage is small, a data write rate is disadvantageously reduced.